Many image compression techniques utilize the DPCM method. In this method the image is transmitted in a raster scanned fashion. Based on the previously transmitted information, a prediction or estimate of the value of the next pixel is made. Only the difference between the predicted value and actual value is actually transmitted. The residuals have a much smaller range of values than the actual pixel values and can be transmitted with many fewer bits per pixel than the actual pixel values can be transmitted. Since both the transmitter and receiver use the same method of prediction based on information which both have available, the receiver will make the same prediction as the transmitter and can faithfully reconstruct the image.
The predictor used in the classical DPCM method is a linear combination of some subset, called the support, of the values already transmitted. One common such subset is the just prior pixel on the same line, together with the three nearest pixels on the previous line. The problem with the linear combination technique is that it behaves poorly on edges. In some work, it has been proposed that the coefficients making up the linear combination be allowed to vary based on the information already transmitted, making the technique slowly adaptive. This still does not respond quickly enough to rapid changes in the image, and edges in the image still cause large residuals which degrade the performance of the compression technique.
It is seen then that there exists a need for an improved method of pixel value estimation, particularly for use in image compression, which overcomes the problems associated with the prior art.